Coral bleaching

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Two images of the Great Barrier Reef showing that the warmest water (top picture) coincides with the coral reefs (lower picture), setting up conditions that can cause coral bleaching.

Coral bleaching is the loss of color of corals, due to stress-induced expulsion of symbiotic unicellular algae or due to the loss of pigmentation within the algae[1]. The corals that form the structure of the great reef ecosystems of tropical seas depend on a symbiotic relationship with photosynthesizing unicellular algae called zooxanthellae that live within their tissues. Zooxanthellae give coral its particular coloration, depending on the clade living within the coral. Under stress, corals may expel their zooxantheallae, which leads to a lighter or completely white appearance, hence the term "bleached". [2]

Coral bleaching is a vivid sign of corals responding to stress which can be induced by any of:

High sea surface temperature (SST) coupled with high irradiance is known to be the primary factor in summer coral bleaching. Wind, exposure at low tide, and weather conditions can contribute to coral bleaching. [8] The US National Oceanic and Atmospheric Administration (NOAA) monitors for bleaching "hot spots," areas where SST rises 1 degree Celsius or more above the long-term monthly average. Some argue this system detected the massive 1998 bleaching event that was worldwide.[9][10] This corresponded to an El Niño event. At the same time, NOAA Coral Bleaching "Hotspot" program uses a 50k satellite resolution at nighttime, which some argue covers too large of a spatial area and does not incorporate the max SSTs occurring usually around height of daytime, noon.

Bleached corals
Healthy corals

Once bleaching begins, corals tend to continue to bleach even if the stressor is removed. If the coral colony survives, it often requires weeks to months for the remaining symbiont population to reach a normal density [11]. Following bleaching, corals may be recolonised by the same species of zooxanthellae, or by a different species. Different types of zooxanthellae respond differently to environmental conditions and may be more resistant to coral bleaching than other species. Some corals are known to host multiple clades of zooxanthellae within an individual coral. [12]. Ability to withstand stress and bleaching and ability to recover from a bleaching event varies greatly across coral species. Large massive corals, such as Porites lobata is able to withstand extreme temperature shocks, while fragile branching corals, such as Acropora spp. are far more susceptible to thermal stress following a bleaching event [13]. Recent research has also shown that corals consistently exposed to low levels of stress may in fact be more resistant to bleaching. Factors that protect against mass coral bleaching are bleaching resistance, coral tolerance, reef recovery. Due to the patchy nature of bleaching, local climatic conditions such as shade or a stream of cooler water can reduce the risk of bleaching. Also, the health and genetics of both the coral and its zooxanthellae can influence the risk of bleaching.[14]

Other reef creatures have symbiotic zooxanthellae, which they may also expel under stressful conditions. Bleaching stress is also exhibited by soft corals, giant Tridacna clams and some sponges.

The Great Barrier Reef along the northeast coast of Australia suffered two mass coral bleaching events in the summers of 1998 and 2002, and also in the southern GBR in 2006. While most reef areas recovered with relatively low levels of coral death, some locations suffered severe damage, with up to 90% of corals killed [15] Based on IPCC 2007 assessment, coral reefs will be highly susceptible to more frequent bleaching events, with the additional problem of acidification due to the increasing carbon dioxide levels in the next twenty to thirty years (IPCC, 2007[16]; Fischlin et al., 2007[17]; Nicholls et al., 2007[18]).

Intergovernmental Panel on Climate Change in its Fourth Assessment Report, Working Group II, regional Chapter on Australia and New Zealand (Hennessy et al., 2007 [19]):

"Greatest threat to GBR (i) rising sea temperatures, which are almost certain to increase the frequency and intensity of mass coral bleaching events (ii) ocean acidification, which is likely to reduce the calcifying ability of key organisms such as corals. GBR has experienced eight-mass bleaching events since 1979 (1980, 1982, 1992, 1994, 1998, 2002, and 2006) The most widespread and intense events occurred in the summers of 1998 and 2002, with about 42% and 54% of reefs affected, respectively (Done et al. 2003) Berkelmans 2004). The effects from thermal stress are likely to be exacerbated under future scenarios by the gradual acidification of the world’s oceans.

Even under a moderate warming scenario (B1 to A1T, 2°C by 2100, IPCC, 2007, Table SPM.3, p.13[20]), corals on the GBR are very likely to be exposed to regular summer temperature that exceed the thermal thresholds observed over the past 20 years (e.g. Done et al. 2003)."

Other coral reef provinces have been permanently damaged by warm sea temperatures, most severely in the Indian Ocean. Up to 90% of coral cover has been lost in the Maldives, Sri Lanka, Kenya and Tanzania and in the Seychelles.

Evidence from extensive research in the 1970’s of thermal tolerance in Hawaiian corals and of an oceanic warming trend led researchers in 1990 to make predictions of mass occurrences of coral bleaching would be found throughout the Hawaiian Island archepelego. The first major bleaching occurred in 1996 and the second in 2002. [21]

Coral in the south Red Sea has been found not to bleach despite summer water temperature up to 34°C: see Red Sea#Climate.

Warming trends can be seen in coral reef areas world wide. Unfortunately predictions for future decades suggest increasing severity and frequency (IPCC Fourth Assessment Report: Fischlin et al., 2007, Figure 4.4, Table 4.1[22]; Nicholls et al., 2007[23]; Hennessy et al., 2007[24]; IPCC, 2007, Figure SPM.2, p.16[25]).

Contents

[edit] Pathogen infection

Bioerosion (coral damage) such as this may be caused by coral bleaching.[26]

In 1996, Kushmaro, et al. [27] reported that the agent for bleaching of the coral Oculina patagonica in the Mediterranean Sea was an infectious bacteria attacking the symbiotic algae. The agent has been later identified as Vibrio shiloi. The pathogen is infectious only during warm periods; therefore, global warming would increase the occurrence of conditions that promote the spread of infection.Elevated temperature increases the virulence of V. shiloi, which become able to adhere to a beta-galactoside-containing receptor in the mucus on the surface of the host coral.[28][29] Then, V. shiloi penetrates and multiplicates into epidermis of the coral, and finally transforms into a “viable but non-culturable” (VBNC) state, and produces both heat-stable and heat-sensitive toxins, affecting zooxanthellae by inhibiting their photosynthetic activity and causing their lysis. During the summer of 2003, coral reefs in the Mediterranean Sea appeared to gain resistance to the pathogen, and further infection was not observed [30]. The main hypothesis for the emerged resistance is the presence of symbiotic communities of bacteria living with the corals. One species capable of lysing V. shiloi has gained prominence. This hypothetical bacteria has not yet been identified.

[edit] See also

Ove Hoegh-Guldberg (biologist)

[edit] Notes and references

  1. ^ Dove, SG & O Hoegh-Guldberg. 2006. Coral bleaching can be caused by stress (due to rise in water temperature, diesease, or ceartain chemicals). The cell physiology of coral bleaching. In Coral Reefs & Climate Change: Science and Management. JT Phinney, W Skirving, J Kleypas & O Hoegh-Guldberg, eds. American Geophysical Union. pp 1–18.
  2. ^ Hoegh-Guldberg 1999
  3. ^ "REEF ‘AT RISK IN CLIMATE CHANGE’". http://www.coralcoe.org.au/news_stories/climatechange.html. Retrieved on 2007-07-12. 
  4. ^ Fitts 2001
  5. ^ Climate Change and The Great Barrier Reef — A Vulnerability Assessment — GBRMPA
  6. ^ Hoegh-Guldberg et al. "Coral Reefs Under Rapid Climate Change and Ocean Acidification". Science 14 December 2007: Vol. 318. no. 5857, pp. 1737–1742 DOI: 10.1126/science.1152509
  7. ^ The Starving Ocean: Mass Coral Bleaching
  8. ^ Anthony, K. 2007; Berkelmans
  9. ^ "NOAA Hotspots". http://coral.aoml.noaa.gov/pipermail/coral-list/2006-October/003757.html. 
  10. ^ "Pro-opinion of NOAA Hotspots". http://www.osdpd.noaa.gov. 
  11. ^ Jokiel 1978
  12. ^ "The Future of Coral Reefs by Nancy Knowlton ok.." (PDF). http://chge.med.harvard.edu/education/course_2007/topics/02_28/documents/knowlton.pdf. Retrieved on 2007-03-02. 
  13. ^ Baird and Marshall 2002
  14. ^ A Reef Manager’s Guide to Coral Bleaching. Townsville, Australia: Great Barrier Reef Marine Park Authority,. 2006. ISBN 1 876945 40 0. http://www.gbrmpa.gov.au/corp_site/info_services/publications/misc_pub/a_reef_managers_guide_to_coral_bleaching. 
  15. ^ Climate Change and The Great Barrier Reef — A Vulnerability Assessment — GBRMPA; Baird and Marshall 1998
  16. ^ IPCC, 2007. Summary for policymakers. In: Parry, M.L., Canziani, O.F., Palutikof, J.P., van der Linden, P.J. & Hanson, C.E. (eds.), Climate change 2007: Impacts, adaptation and vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel of Climate Change (IPCC). Cambridge University Press, Cambridge, UK, pp. 7-22. http://www.ipcc.ch/pdf/assessment-report/ar4/wg2/ar4-wg2-spm.pdf
  17. ^ Fischlin, A., Midgley, G.F., Price, J.T., Leemans, R., Gopal, B., Turley, C., Rounsevell, M.D.A., Dube, O.P., Tarazona, J. & Velichko, A.A., 2007. Ecosystems, their properties, goods and services (chapter 4). In: Parry, M.L., Canziani, O.F., Palutikof, J.P., van der Linden, P.J. & Hanson, C.E. (eds.), Climate change 2007: Impacts, adaptation and vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel of Climate Change (IPCC). Cambridge University Press, Cambridge, UK, pp. 211-272. http://www.ipcc.ch/pdf/assessment-report/ar4/wg2/ar4-wg2-chapter4.pdf
  18. ^ Nicholls, R.J., Wong, P.P., Burkett, V., Codignotto, J., Hay, J., McLean, R., Ragoonaden, S. & Woodroffe, C.D., 2007. Coastal systems and low-lying areas (chapter 6). (vii, 357973): 315-357. http://www.ipcc.ch/pdf/assessment-report/ar4/wg2/ar4-wg2-chapter6.pdf
  19. ^ Hennessy, K., Fitzharris, B., Bates, B.C., Harvey, N., Howden, M., Hughes, L., Salinger, J. & Warrick, R., 2007. Australia and New Zealand (chapter 11). (vii, 357973): 507-540. http://www.ipcc.ch/pdf/assessment-report/ar4/wg2/ar4-wg2-chapter11.pdf
  20. ^ IPCC, 2007. Summary for policymakers. In: Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M. & Miller, H.L. (eds.), Climate change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 1-18. http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-spm.pdf
  21. ^ Hokiel, Paul J.. "Climate Change and Hawaii’s Coral Reefs". Hawaii Coral Reef Monitoring and Assesment Program, US Fish and Wildlife Service. http://www.fws.gov/coralreef/proceedings/Day%202%20PDF/5-Athline%20Clark.pdf Retrieved on February 9,2009
  22. ^ Fischlin, A., Midgley, G.F., Price, J.T., Leemans, R., Gopal, B., Turley, C., Rounsevell, M.D.A., Dube, O.P., Tarazona, J. & Velichko, A.A., 2007. Ecosystems, their properties, goods and services (chapter 4). In: Parry, M.L., Canziani, O.F., Palutikof, J.P., van der Linden, P.J. & Hanson, C.E. (eds.), Climate change 2007: Impacts, adaptation and vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel of Climate Change (IPCC). Cambridge University Press, Cambridge, UK, pp. 211-272. http://www.ipcc.ch/pdf/assessment-report/ar4/wg2/ar4-wg2-chapter4.pdf
  23. ^ Nicholls, R.J., Wong, P.P., Burkett, V., Codignotto, J., Hay, J., McLean, R., Ragoonaden, S. & Woodroffe, C.D., 2007. Coastal systems and low-lying areas (chapter 6). (vii, 357973): 315-357. http://www.ipcc.ch/pdf/assessment-report/ar4/wg2/ar4-wg2-chapter6.pdf
  24. ^ Hennessy, K., Fitzharris, B., Bates, B.C., Harvey, N., Howden, M., Hughes, L., Salinger, J. & Warrick, R., 2007. Australia and New Zealand (chapter 11). (vii, 357973): 507-540. http://www.ipcc.ch/pdf/assessment-report/ar4/wg2/ar4-wg2-chapter11.pdf
  25. ^ IPCC, 2007. Summary for policymakers. In: Parry, M.L., Canziani, O.F., Palutikof, J.P., van der Linden, P.J. & Hanson, C.E. (eds.), Climate change 2007: Impacts, adaptation and vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel of Climate Change (IPCC). Cambridge University Press, Cambridge, UK, pp. 7-22. http://www.ipcc.ch/pdf/assessment-report/ar4/wg2/ar4-wg2-spm.pdf
  26. ^ Ryan Holl (17 April 2003). "papers/Bioerosion.htm Bioerosion: an essential, and often overlooked, aspect of reef ecology". Iowa State University. http://www.biology.iastate.edu/intop/1Australia/Australia papers/Bioerosion.htm. Retrieved on 2006-11-02. 
  27. ^ Kushmaro et al. Bacterial infection and coral bleaching, Nature 380:396 (1996)
  28. ^ Rosenberg, E. and Y. Ben Haim [2002], Microbial Diseases of Corals and Global Warming. Environmental Microbiology 4: 318-326.
  29. ^ Sutherland, K.P., J. Porter and C. Torres [2004], Disease and Immunity in Caribbean and Indo-pacific Zooxanthellate Corals. Marine Ecology Progress Series 266: 273-302.
  30. ^ Reshef et al., The coral probiotic hypothesis Env Microbiolgy, 8:2066 (2006)

[edit] External links

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